One of the most troublesome steps in the manufacture of wood veneers, and other items of like physical characteristics, is that of drying the item. To have a commercially feasible method and apparatus, drying must be accomplished quickly and economically, and with minimal risk of damage. In the prior art, these three objectives have not coincidentally been attainable.
Checking, scorching and lowering of fiber strength are three results of improper drying that cause damage to the item being dried. Checking is the presence of cracks in the fissures of the dry wood. It is principally caused by removing water so rapidly from the outermost layers that they undergo physical contraction that sets up uneven stresses in those layers. This condition often occurs when wood is dried by direct hot air means. Scorching is caused when high surface temperatures, such as at above 270.degree. F. at atmospheric pressure, are created on the material being dried in an effort to promote quick water removal. This is particularly troublesome when using infrared heating at high temperatures. Scorch is not usually present when drying is done by steaming, but the steaming process has its own problems, one of which is the possibility of lowering the fiber strength, due to hydrolyzing action of the moisture.
Air drying is a well known method of drying woods. It is accomplished by simply allowing the wood to rest in atmospheric conditions, or by blowing heated air over it. However, air drying is a very slow process when done under atmospheric conditions. Forced or heated air drying is also faster, but the temperatures must be controlled to prevent case hardening. It requires a significant level of energy over a long period of time. Steam drying is faster than air drying under normal conditions, and requires a higher level of energy input.
Some prior art methods have included either or both of the above concepts carried out under vacuum conditions, for various reasons, all said to enhance their operation. These have been batch processes, not designed for the continuous flow processing that is necessary when drying veneers under commercial circumstances.
Various sources of heat have also been used in the prior art. Air or steam is heated by various means and circulated over the wood in some of the systems. In others, the wood is placed in contact with a heated body, or the chamber is heated from inside or outside. Instances of heating by infrared energy are also present in the prior art, but not in veneer dryers or the like.